1. Field of the Invention
The present invention relates to a method for manufacturing a surface acoustic wave element, and, more specifically, relates to a method for manufacturing a surface acoustic wave element, such as a filter or a resonator, utilizing surface acoustic waves propagating along a surface of a piezoelectric substrate.
2. Description of the Related Art
Previously, in a small mobile communication system in which the frequency difference between a transmission band and a reception band is small, for example, a surface acoustic wave filter using a piezoelectric substrate having a high electromechanical coupling coefficient, such as lithium tantalate (LiTaO3; LT) or lithium niobate (LiNbO3; LN), has been used in order to secure the attenuation in the reception band. However, since the temperature coefficient of frequency (TCF) of an LT substrate or an LN substrate is high, the interval between the transmission band and the reception band becomes very small considering the manufacturing variation. Therefore, an improvement of temperature characteristics has been desired.
As a measure for improving the temperature characteristics, a structure has been proposed in which a support member having strength and elasticity higher than those of a piezoelectric substrate is bonded to the piezoelectric substrate.
As a method for manufacturing a surface acoustic wave element having such a structure, a structure as shown in the cross sectional view of FIGS. 5A-5C, for example, has been proposed in which a relatively thick piezoelectric substrate 111A and a silicon substrate 112A are bonded to each other, and then each substrate 111A and 112A is cut and ground (cut and removed portions 111C and 112C in FIGS. 5B and 5C) so that a piezoelectric substrate 111B and a silicon substrate 112B that are thinned to a desired thickness are bonded to each other (e.g., Japanese Unexamined Patent Application Publication No. 2004-297693).
Moreover, a method for manufacturing a substrate for a surface acoustic wave element shown in the cross sectional views of FIGS. 6A-6D has been proposed. More specifically, a first substrate 201 and a second substrate 202 that are different in linear expansion coefficient are cleaned as shown in FIG. 6A, and then the first substrate 201 and the second substrate 202 are bonded to each other to form a bonded substrate 206 as shown in FIG. 6B. Then, as shown in FIG. 6C, slits 207 are formed in the side of the first substrate 201 of the bonded substrate 106, the bonded substrate 206 is heated for increasing the bonding strength, and, by a thermal stress caused by the heating treatment, the first substrate 201 is divided along with the slits 207 as shown in FIG. 6D (e.g., Japanese Unexamined Patent Application Publication No. 2002-217666).
In the manufacturing method as shown in FIGS. 5A-5C, electrode patterns, such as an IDT electrode (IDT: interdigital transducers), need to be formed on the surface of the thinned piezoelectric substrate. The piezoelectric substrate and the support substrate are different from each other in the linear expansion coefficient, and the piezoelectric substrate is thinned. Thus, in a process for forming electrode patterns, the piezoelectric substrate is likely to curve in response to temperature changes. When the piezoelectric substrate curves, problems such that the electrode pattern formation accuracy decreases and the piezoelectric substrate itself is damaged are likely to occur.
In the manufacturing method as shown in FIGS. 6A-6D, it is difficult to accurately control the depth, the width, or the angle of the slits, and thus, when heat is applied, damage such as fracture or cracks, is likely to occur in the substrate. Moreover, even when the first substrate can be completely cut, it is difficult to accurately align the patterns in a process for forming the electrode patterns on the surface of the first substrate due to the slits in the surface of the first substrate.